Low concentrations of sulfur contaminants occur in gas streams such as coke oven gas, natural gas, and the tail gas from the Claus process. The Claus process, which is widely used in petroleum refineries to convert hydrogen sulfide by-product to sulfur, generates large volumes of waste tail gas. Direct discharge of this gas, which contains residual H.sub.2 S, to the atmosphere results in pollution levels unacceptable to an increasing number of communities.
A highly effective process for removing low concentrations of hydrogen sulfide from contaminated gas streams is the Stretford process. In this method, the gas stream is contacted with aqueous sodium carbonate/bicarbonate solution that contains pentavalent vanadium and anthraquinonedisulfonic acids (ADA). The hydrogen sulfide is oxidized to sulfur with accompanying reduction of the vanadium and ADA. After the sulfur is removed, the used aqueous solution now containing tetravalent vanadium and reduced ADA is regenerated by oxidation with air and recycled to the gas absorber. The ADA also acts as a catalyst for the regeneration step. The net reaction is the indirect oxidation of hydrogen sulfide to form water and sulfur, which is recovered. The reactions can be represented as follows: ##STR1##
The process performs its intended function of removing hydrogen sulfide from waste gas streams or other gas streams such as natural gas quite effectively. Nonetheless, the process suffers a drawback. During each cycle of the process, a small percentage of sulfide is converted irreversibly to thiosulfate and to a lesser extent sulfate. Other waste materials, such as sulfites, mercaptans and dissulfides may also be present. The accumulation of thiosulfate particularly reduces the solubility of vanadium and ADA in the solution, and decreases the rate of oxidative regeneration. Generally, thiosulfate levels in excess of about 25 wt % are very undesirable because both ADA and vanadate salts are largely salted out at these levels. To maintain the thiosulfate and sulfide salts at an acceptable level of 20-25 wt %, a continuous purge becomes necessary. This results in a loss of valuable chemicals: ADA,sodium vanadate, and sodium carbonate. More important, however, high thiosulfate concentrations present a serious problem for disposing of this purge stream because of their high chemical oxygen demand(COD).
Thiosulfate is stable and has high water solubility. The methods for its destruction and/or removal from the process stream are costly. Proposed processes for disposal of this effluent have included evaporation, incineration, and even biodegradation. All are unattractive economically, however.